1. Field of the Invention
This invention relates to a laser apparatus including a laser oscillator for producing a laser output by exciting a gas or a solid medium, or a laser machine, namely laser beam machine, for a machining operation such as cutting or welding, using the laser output from the laser oscillator.
2. Description of the Related Art
Generally, a laser apparatus mainly comprises a laser oscillator, an optical transmission system for transmitting and condensing light, a device for driving a work, auxiliary equipment and a control unit for controlling these component parts. An ordinary laser apparatus used for the laser cutting operation, for example, includes a laser oscillator, a machining table driven in X, Y and Z directions, an optical transmission system having a reflector, a laser machining head for condensing the laser light, an assist gas supply system for supplying an assist gas to the laser head, a cooling water circulation unit for cooling the laser oscillator, the optical transmission system and the machining head, a dust collector, a laser medium supply system and a CNC (computer numerical control) for controlling these component parts. Further, the laser oscillator is configured of a laser gas circulation cooling system, a resonator, a discharge tube system, a power supply for supplying the exciting energy to the discharge tube and a laser control unit for controlling these component parts. In a conventional technique, the CNC may double as a laser control unit.
This laser apparatus has various sensors. The most common sensor is a laser power sensor. When a part of the laser light recovered from a rear mirror is radiated on the laser power sensor, the laser power sensor measures the temperature increase thereby to calculate the heat flow rate and detect the laser output. The laser apparatus also includes sensors for measuring the temperature, pressure, flow rate, current and/or voltage of various media. Based on the measurements of these sensors, the laser apparatus appropriately outputs an alarm and performs a predetermined feedback control operation. In this way, the laser apparatus outputs a stable laser output and realizes a stable laser machining operation in spite of various disturbances.
The current tendency is toward a further increased output of the laser oscillator used for the laser apparatus. At various points of the laser apparatus, therefore, a considerable difference arises between the temperature immediately following the start operation of the oscillator and the temperature during the steady operation thereof. The laser apparatus is a complicated and precise device having a multiplicity of subsystems as described above, and the characteristics of these subsystems may change with temperature as described above.
The central portion of the condensing lens in the neighborhood of the machining head is heated, for example, by the high-output laser transmitted therethrough, resulting in a temperature difference between the central portion and the peripheral portion of the condensing lens. The central portion of the lens expands and the refractive index of the central portion of the lens undergoes a change, thereby leading to what is called a thermal lens effect in which the actual focal length is changed. As a result, the optimum position of the lens with respect to the surface of the work to be machined may be different at the time of starting the laser machining operation and during the steady operation.
In the laser oscillator, on the other hand, the laser gas constituting a laser medium increases in temperature due to the discharge. As a result, the laser gas expands, and therefore the laser gas supply system is disturbed. Thus, the laser gas pressure control is upset thereby causing the irregularities of the laser output and discharge voltage. At the same time, the discharge tube and the discharge electrodes are heated, and therefore the discharge characteristic is changed. Further, the resonator body is distorted by thermal expansion, so that the laser output at the time of laser activation and during the steady operation may be different.
To cope with this problem, Japanese Unexamined Patent Publication No. 2000-94173 discloses a configuration in which the temperature of a condensing lens is measured thereby to control the optimum lens position due to the thermal lens effect. Also, Japanese Unexamined Patent Publication No. 10-229229 discloses a method in which the laser gas temperature is detected, and, after warming an oscillator up to the laser gas temperature at which the laser light is stabilized, the laser light is radiated thereby to improve the laser output control. In this case, the temperature of each component element of the oscillator is measured while at the same time securing the required warm-up operation thereby to obtain a stable output.
Further, Japanese Unexamined Patent Publication No. 7-106678 discloses a configuration in which the laser gas temperature and the discharge electrode temperature are measured and, based on these temperatures, the laser apparatus is controlled.
As described above, the temperature of each component element of the laser apparatus is measured and based on the characteristic change corresponding to the measured temperature, the laser apparatus is controlled. In this case, the laser output can be stabilized and the stable laser machining operation can be realized within a comparatively short time after starting the laser.
Installation of the temperature sensors on the component elements of the laser apparatus such as the discharge electrodes and the condensing lens, however, requires a physical space. Also, even in the case where the installation is possible, the temperature of a specified component element such as the discharge electrodes impressed with a high voltage and the central portion of the condensing lens cannot be easily measured, and therefore it is not an easy matter to continuously detect the temperature accurately while the laser apparatus is driven. Further, these sensors, of course, are costly, and a temperature sensor of high response rate that can produce a real output within one millisecond from an output command such as for the laser is very expensive. The temperature sensor having such a high response rate cannot be realistically used for every part of the laser apparatus.
To obviate this problem, Japanese Unexamined Patent Publication No. 10-135542 discloses a configuration in which in order to control the laser output in accordance with the characteristic change with the temperature of a laser medium, the effect of the temperature rise of the laser gas providing a laser medium is produced as a parameter from the laser output time, the laser output immediately preceding to the present time point and the off time.
In view of the fact that a high-performance high-output laser apparatus currently available has a high-performance heat exchanger as a laser gas cooler, the temperature of the laser gas supplied to the laser oscillation area is very stable. Also, the laser apparatus has a strong laser-gas blower, so that the laser gas in the laser oscillation area is replaced entirely within one millisecond. Although the temperature of the laser gas can be stabilized within several milliseconds after starting the laser output, however, the component elements of the laser apparatus such as the optical parts making up the discharge electrodes, the discharge tube and the resonator cannot be cooled within as short a time as the laser gas can. During the period of several seconds to several minutes after the start of the laser output, therefore, the characteristics of these component elements undergo a change with temperature. For this reason, the laser apparatus cannot be actually controlled properly in accordance with the temperature change of each specified component element based on the method disclosed in Japanese Unexamined Patent Publication No. 10-135542.
In these patent publications, the cooling process is not sufficiently taken into consideration, and therefore the laser apparatus disclosed in these patent publications cannot be used for the continuously-changing laser control operation. It is for this reason that these patent publications relates to the pulse oscillation. Actually, the high-output laser often transfers to a machining operation with a continuous low output or a low-duty pulse output immediately after the machining operation with the rated maximum output. In such a case, in spite of the continued laser output, the laser apparatus as a whole enters a cooling process. If the control is operated in accordance with the time when the laser output is off, therefore, a higher accuracy cannot be achieved. This problem is shared by Japanese Unexamined Patent Publication No. 7-106678.
The object of this invention is to provide a laser apparatus which obviates the above-mentioned problem of the prior art and can operate stably for a long time without any temperature sensor as a component element.